Wavelength conversion crystal and method for generating laser beam, and apparatus for generating laser beam

ABSTRACT

As a wavelength conversion crystal whose double refraction index is controllable, a crystal represented by a formula (I), M 1   x M 2   1−x Ca 4 O(BL 3 ) 3 , where each of M 1  and M 2  represents one or more types of different rare earth elements and 0&lt;x&lt;1, is used, and as a novel means for second harmonics generation, a nonlinear optical crystal represented by a formula (II), Gd x Y 1−x Ca 4 O(BO 3 ) 3 , where 0.01≦x≦0.35, is used to generate second harmonics.

This is a 371 application of PCT/JP99/01598 filed Mar. 29, 1999.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a wavelength conversion crystal, and amethod and apparatus for generating a laser beam. More particularly, thepresent invention relates to a novel wavelength conversion crystaluseful as a nonlinear optical crystal, and a method and apparatus forgenerating a laser beam.

2. Description of the Related Art

With the drastic revolution of laser technologies in recent years, ithas become a major challenge to perform the wavelength conversion of anear-infrared solid-state laser beam using a nonlinear optical crystal.

A solid-state laser has a narrow spectral bandwidth and stable output,and is easily maintained and feasible for miniaturization, so that it isattracting attention as a means for laser processing and laser-basedmedical treatments, and also for applications such as surface reformingand optical information processing. In order to capitalize on thesebeneficial properties of such solid-state lasers, the wavelengthconversion technologies have become increasingly important.

An ideal nonlinear optical crystal for such wavelength conversion isrequired to have; i) a large nonlinear optical constant; ii) a shortabsorption edge; and iii) an adequate double refraction index. Also, asa crystal, it is further desired to have; iv) superior mechanicalproperties; from a practical point of view.

The term “iii) an adequate double refraction index” is a doublerefraction index that satisfies its non-critical phase matchingcondition under which the wavelength conversion is performed mostefficiently. When the double refraction index is smaller than the idealvalue, the wavelength conversion would become impossible, and when it islarger, the conversion efficiency would degrade since such a large valueresults in the departure from the non-critical phase matching condition.

Nonlinear optical crystals have been studied from various viewpoints,and among them, calcium oxyborate-type (COB) crystals are attractingattention.

For example, the non-linearity had been found in GdCa₄O(BO₃)₃:GdCOB byAka et al., and the growth and optical properties of its single crystalhad been reported in 1996. It had been found that this GdCOB;

can be grown by the Cz method, and is non-water-soluble;

has a Vickers hardness of approximately 600 (as hard as quartz);

has d_(off) (at 1064 nm) of 1.3 pm/V (about 3.4 times of KDP);

has a phase matching threshold wavelength of 840 nm; and

is incapable of generating third harmonics of Nd:YAG.

However, the major drawback of this GdCOB is in the fact that its doublerefraction index is as small as 0.033.

That is, although this GdCa₄O(BO₃)₃(GdCOB) crystal is easy to grow andsuperior in its mechanical properties, the wavelength it can possiblygenerate through wavelength conversion is long because its doublerefraction index is small. Accordingly, the inventors of the presentinvention have discussed a means to increase this double refractionindex, and found that when Gd in a GdCa₄O(BO₃)₃(GdCOB) crystal isreplaced by Y, its double refraction index increases. As a result, whilea GdCOB crystal can only generate second harmonics of an Nd:YAG laser,the YCOB in which Gd is substituted by Y can generate third harmonics ofan Nd:YAG laser.

The inventors of the present invention have already proposed this newlydiscovered YCOB crystal in a concretive manner.

However, since the arbitrary control over the double refraction indicesof COB crystals had been believed to be impossible, the inventors of thepresent invention extended their discussion over COB as a nonlinearoptical crystal for wavelength conversion, and set their objectives toprovide a novel technical approach which allows the optimization controlover the double refraction indices, as a critical requirement forespecially wavelength conversion.

In addition, for wavelength conversion crystals, the conversion tosecond harmonics has also presented a critical problem.

This problem stems out from the fact that, LBO(LiB₃O₅) crystal currentlyused as a wavelength conversion crystal for generating second harmonicsof Nd:YAG laser has quality problems and its growing cost is high sincethis LBO crystal is a water-soluble crystal so that it does not providea sufficient life span and reliability, and in addition, since it has tobe used at a temperature of as high as 148° C., and the crystal growthis difficult. Therefore, there has been a demand for the development ofa wavelength conversion crystal, which can substitute LBO crystal forgeneration of second harmonics of Nd:YAG laser. Especially, there hasbeen a strong demand for a wavelength conversion crystal which cangenerate second harmonics of Nd:YAG laser in a manner in which thenon-critical phase matching condition can be satisfied even under a roomtemperature, which has not yet been implemented in the past.

Furthermore, various types of optical elements for wavelength conversionhave previously been proposed. For example, a wavelength conversionelement has been used in an ultraviolet laser beam oscillator forconverting an infrared beam into an ultraviolet beam. However, in theprior art, a large number of optical elements were required, resultingin complexity in the optical system, and thus, making it difficult toconstruct a small laser beam oscillator.

Moreover, there had been proposed a crystal element, which allows thegeneration of second harmonics as well as the oscillation of an infraredlaser beam with this one same element. However, it has been difficult toobtain a crystal element capable of generating up to third harmonics.

Accordingly, a novel multifunctional laser beam generator has beendesired, which, as a single optical element, has multi-functionality,and is capable of generating second and third harmonics, and is alsocapable of being implemented in a small ultraviolet laser beamgenerator.

SUMMARY OF THE INVENTION

The present invention first provides a wavelength conversion crystal,which is expressed in the following formula (I); M¹ _(x)M²_(1−x)Ca₄O(BO₃)₃; where each of M¹ and M² represents one or more type ofdifferent rare earth elements, and 0<X<1.

More particularly, the present invention provides also a wavelengthconversion crystal in which M¹ and M² of the above formula are selectedfrom a group comprising Gd, Y, La and Lu.

As stated above, the present invention is based on an X-ray diffractionobservation made on a sintered body, which verified that, for GdCOBexpressed by GdCa₄O(BO₃)₃, it is not only possible to replace the Gdwith Y, it is also possible to introduce the rare earth elements such asLu and La etc. into the Gd site so as to change the lattice constant.Since there is a correlation between a lattice constant and a refractiveindex, the change in the lattice constant would mean a change in thedouble refraction index of the crystal. Accordingly, the discussion wasfurther extended, and it was found that the double refraction indexcould be arbitrary controlled by changing the ratios of Gd, Y, Lu andLa, and this discovery provided integrity to the present invention. Thatis, the double refraction index changes in the order of, for example,Lu>Y>Gd>La. As a result, an optimal double refraction index can beobtained for any arbitrary wavelength from third harmonics (355 nm) tosecond harmonics (532 nm) of Nd:YAG laser, so that the non-criticalphase matching condition can always be satisfied.

The present invention secondly provides a novel means for generatingsecond harmonics, which allows an optimization control over the doublerefraction index of a COB crystal to replace the LBO crystal that haspreviously been used for second harmonics generation.

That is, the present invention provides a nonlinear optical crystal forsecond harmonics generation, which is represented by the followingformula (II); Gd_(x)Y_(1−x)Ca₄O(BO₃)₃; where 0.01 ≦X≦0.35.

The present invention also provides a method for generating laser beamwherein the laser beam is sent through a nonlinear optical crystalrepresented by this formula (II) to convert it into second harmonics,and an apparatus for generating laser beam comprising this crystal as ameans for generating second harmonics.

Furthermore, the present invention also provides a laser beam generatorhaving a nonlinear optical crystal as a means for generating thirdharmonics.

The present invention thirdly provides, as a novel laser beam generatorcapable of generating the fundamental wave of laser, andwavelength-converted beams of second and third harmonics, a laser beamgenerator comprising a nonlinear optical crystal of calciumoxyborate-type crystal containing Gd and Y with Yb or Nd doped thereto,and wherein this crystal element is configured to be able to perform thelaser oscillation of the fundamental wave and the generation of secondand third harmonics laser beams.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the relationship between the compositional variables“x” and phase matching angles in the generation of second harmonics.

FIG. 2 illustrates the relationship between the external angles andnormalized second harmonics outputs.

FIG. 3 is a schematic diagram of an exemplary method for generatingsecond harmonics and apparatus configuration therefor.

FIG. 4 is a schematic diagram of an exemplary configuration of anultraviolet laser beam generator by semiconductor laser excitation.

FIG. 5 is a schematic diagram of an exemplary configuration of anotherlaser beam generator.

FIG. 6 is a schematic diagram of an exemplary configuration of yetanother laser beam generator.

FIG. 7 is a perspective view of an apparatus for crystal growth with apart of it removed.

FIG. 8 is a diagram illustrating portions of grown crystal forcompositional analysis and ratios of Gd/Y.

FIG. 9 is a diagram showing results of calculations of non-criticalphase matching wavelength in the z-axis direction.

FIG. 10 is a diagram showing an exemplary relationship betweencompositions (x) and phase matching angles in generating thirdharmonics.

The descriptions of reference numerals shown in FIG. 3 are as follows;1: Nd:YAG infrared laser beam oscillator, 2: near-infrared beam, 3:convex lens, 4: nonlinear optical crystal, 5: green laser beam, 6:convex lens.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention has the features and background as set forthabove, and embodiments thereof will now be explained in greater detail.

In the wavelength conversion crystal of the present invention, which isexpressed in the above formula (I), the rare earth elements of M¹ and M²are different from each other. These may be rare earth elementsincluding Gd, Y, La and Lu etc.

Although the method for growing crystal is not limited to a particularmethod, it may be grown by, for example, the high frequency inductionheating Cz method.

Also, while the inventors of the present invention attempt to realizethe generation of third harmonics under a non-critical phase matchingcondition at a room temperature and at a rising temperature usingGd_(x)Y_(1−x)Ca₄O(BO₃)₃ whose Gd portion has been replaced, the samething is made possible in the generation of second harmonics (a greenlight beam having a wavelength of 532 nm) using a nonlinear opticalcrystal having a composition of the formula (II).

It should be noted that the compositional ratio “x” in the formula (II)is limited to a range between 0.01 and 0.35. It should be consideredthat, especially at a room temperature, an optimal value of “x” for thegeneration of second harmonics is equal to, or in the proximity of 0.28.

For example, when the proportion of Gd is in the proximity of 28%(x=0.28), it is certain that second harmonics are generated under thenon-critical phase matching condition. FIG. 1 illustrates an exemplarygeneration of second harmonics.

It should be noted that a compositional ratio “x” correlates to a phasematching angle and a temperature at which the crystal is being held. Bycontrolling the temperature, the relationship between the phase matchingangle and compositional ratio “x” can be controlled. For example, thecompositional ratio “x” can generally be smaller when the crystaltemperature is increased.

In the past, the generation of second harmonics was only possible withan LBO crystal heated to 148° C. on a practical level, so that thecrystal of the present invention is a breakthrough as it allows suchgeneration at a room temperature, and by varying the composition, atvarious other temperatures. As for its properties, its stability inrelation to the angle (angle tolerance) is significantly improvedcompared to those of other crystals, and that can be clearly seen fromthe data of this crystal shown in FIG. 2, and from the table below whichis the comparison made with a conventional case. The term “angletolerance” means a range of angle in relation to a 1 cm crystal element,extending between angles that would cause the output of the element tobe reduced to one half.

Table 1

Angle Tolerances of Various Crystals for Second Harmonics

Generation

[unit in mrad cm, and in mrad cm^(½) at NCPM(non-critical phasematching)]

GdYCOB (X-0.28) type 2-NCPM 119 LBO (148° C.) type 1-NCPM 71.9 LBO type-1 9 CLBO type -2 1.7 KD*P type -2 5.0 KTP type -2 15

Since the directions of the fundamental wave (infrared beam,wavelength:1064 nm) and second harmonics (a green light beam) arecompletely identical (walk off angle: 0°), the interaction length islong. It is its characteristic that the incident light beams areconveniently overlapped on the next stage for generating higherharmonics, so that it is advantageous for wavelength conversion.

One more important thing is that, at a room temperature, the secondharmonics can be output to the direction which satisfies thenon-critical phase matching condition of third harmonics when thecomposition of Gd is approximately x=0.28. This was discovered for thefirst time in the world. It is not possible to simultaneously anddirectly generate, from a fundamental wave to third harmonics, but bytaking advantage of this identical directional property, the followingapplications may be contemplated.

1. Only a single body of this crystal is inserted into a resonator inwhich a large number of elements are not desirable, to generate secondharmonics and third harmonics.

2. By doping a laser medium (Nd:neodium or Yb:ytterbium) into thecrystal, and additionally incorporating a laser oscillation function toperform, from laser oscillation to ultraviolet beam generation with theuse of the single element.

A laser oscillator used in the green laser beam generation method of thepresent invention is outlined in FIG. 3. In this apparatus, for example,a near-infrared beam 2 having a wavelength of 1064 nm output from anNd:YAG infrared laser beam oscillator 1 is converged by a convex lens 3,and wavelength-converted to a second harmonics having a wavelength of532 nm by a nonlinear optical crystal 4 of the present invention, andthen controlled in its laser spot diameter by a convex lens 6 to beoutput as a green laser beam 5.

In the laser beam oscillator shown in FIG. 3, in addition to theaforementioned nonlinear optical crystal 4, another nonlinear opticalcrystal of a similar composition is disposed to change the angle of alight incident plane, causing to combine the infrared laser beam of 1064nm and the green laser beam of 532 nm, so that an ultraviolet laser beam(third harmonics)of 355 nm can be generated.

Under a room temperature, the generated ultraviolet laser beam of 355 nmmay cause to generate brownish fogging within the crystal, which maydegrade its conversion efficiency. However, it may vanish in time, ormay be removed by raising the temperature. It was confirmed that suchdegradation of the conversion efficiency does not occur in a case wherethe generation of the ultraviolet laser beam of 355 nm is performedwhile raising the temperature.

For example, an apparatus comprising the above means may be used as alaser beam generator of the present invention.

Although it is not limited, the crystal growth may be performed, forexample, by a high frequency induction heating Cz method.

In relation to a laser beam generator such as the above, since a calciumoxyborate crystal containing Gb and Y has a property as a nonlinearoptical element, which allows a second harmonics to be output under anon-critical phase matching condition in the direction same as thedirection that satisfies the non-critical phase matching condition ofthird harmonics, the present invention takes advantage of this identicaldirectional property, and realizes to perform from the laser oscillationto the ultraviolet beam generation, with the single crystal element bydoping a laser medium (Yb:ytterbium or Nd:neodium) into the crystal, andby adding a laser oscillation function.

The present invention also realizes to make it in a one-chipconfiguration with low reflection loss by an existing diffusion weldingmethod.

For example, in the present invention, by providing the configurationillustrated in FIG. 4, which uses an internal resonator structure, asmall ultraviolet laser beam generator by means of semiconductor laserexcitation can be provided.

In this configuration of FIG. 4, calcium oxyborate (COB) containing Gdand Y, to which Yb is doped, for example, is used. Any suitableexcitation semiconductor laser, reflective coating and wave plate may beused. For example, as for the semiconductor laser for excitation, onewith an oscillation wavelength up to 900 nm may be used, and forreflective coating, one having a multi-layer structure may be used.

Furthermore, in the present invention, it is possible to presentexemplary laser beam generators such as the ones shown in FIGS. 5 and 6that can be manufactured through a diffusion welding technique.

For example, an apparatus having means described above may be used as alaser beam generator of the present invention.

Although the method for growing crystal is not limited, a high frequencyinduction heating Cz method may be used. Yb or Nd may be doped by mixingit into the raw material of COB containing Gd and Y, during the growthof this crystal. The doses of Yb and Nd may be determined inconsideration of a desired property and application, and for example,approximately 0.5-30% or 2-20% may typically be considered. The presentinvention will now be further explained in detail according to thefollowing Examples. However, the Examples are merely illustrative innature and serve as representative examples of the preferredembodiments. Other examples within the scope of the claims are possible.Thus, the following Examples should not be construed to narrow thespirit and scope of the claims.

EXAMPLES Example 1

In the setup shown in FIG. 7, a crystal was grown through a highfrequency induction heating Cz method. As for materials, an oxide of arare earth element (RE₂O₃) was used along with CaCO₃ and B₂O₃.

In an iridium (Ir) crucible having a diameter of 50 mm, the crystal wasgrown in an argon (Ar) ambient at a temperature of, for example, 1510°C.

<1> Gd_(0.48)Y_(0.52)COB

A crystal of Gd_(0.48)Y_(0.52)COB was grown under the followingconditions. Seed Orientation: “a” axis (YCOB crystal) Growth Rate: 7mm/h Rotational Speed: 20 rpm Crystal Length: approximately 45 mm

The crystal grown in a length of 4 cm was divided into 5 portions asshown in FIG. 8, and the compositions of top, middle and bottom portionswere measured by ICP. As a result, all of them were found to have aconstant and homogeneous composition of Gd/Y=0.91.

This result also confirmed that a crystal of Gd_(x)Y_(1−x)Ca₄O(BO₃)₃ isa solid solution.

The non-critical phase matching wavelength in the generation of secondharmonics was found to be 920 nm, which falls between 970 nm of a GdCOBcrystal and 840 nm of an YCOB crystal. The non-critical phase matchingwavelength in “z” axis direction is calculated as shown in FIG. 9. Thissuggests that, between 840 nm and 970 nm, a Gd_(x)Y_(1−x)COB crystal canbe grown to have homogeneous composition with an arbitrary value for“x”, and its double refraction index can also be controlled by the valueof “x”.

<2> La_(x)Gd_(1−x)Ca₄O(BO₃)₃, Y_(x)Lu_(1−x)Ca₄O(BO₃)₃

In the same manner as the above, combinations other than the combinationof Gd and Y were also discussed. As a result, it was found that, as wellas Gd_(x)Y_(1−x)Ca₄O(BO₃)₃, La_(x)Gd_(1−x)Ca₄O(BO₃)₃ andY_(x)Lu_(1−x)Ca₄O(BO₃)₃ are also expected to have a superior wavelengthconversion property in the generation of third harmonics (355 nm) tosecond harmonics (532 nm) of Nd:YAG laser.

Example 2

The crystals were grown through a high frequency induction heating Czmethod. As for materials, Gd₃O₃ and Y₂O₃ are used along with CaCO₃ andB₂O₃.

In an iridium (Ir) crucible, the crystals were grown in an argon (Ar)ambient at a temperature of, for example, 1500° C.

The proportion (x) of Gd in Gd_(x)Y_(1−x)Ca₄O(BO₃)₃ was varied to growcrystals of various compositions with x=0.30 or less.

The crystals obtained were confirmed to have homogeneous compositionsand were solid solutions.

As shown in FIG. 2, in this embodiment, the value of x in the proximityof 0.28 was found to satisfy the non-critical phase matching conditionof a phase matching angle 90° C. at a room temperature for secondharmonics generation (type 2).

Also, in a crystal having a Gd composition in the proximity of x=0.28was confirmed to be capable of outputting second harmonics with anapproximate non-critical phase matching condition in the same directionas the direction that satisfies the non-critical phase matchingcondition for third harmonics generation (type 1) as shown in FIG. 10.

Example 3

Yb or Nd was doped to a Gd_(x)Y_(1−x)COB crystal. This doping wasperformed by mixing Yb or Nd into the raw materials of theGd_(x)Y_(1−x)COB crystal during the crystal growth. The compositionalratio (x) was between 0.15 and 0.30, and the dose of Yb or Nd was 5 to10%.

As a result of the evaluation performed on the crystal as an element forgenerating harmonics, the Yb/Nd-doped, Gd-and-Y-containing COB, wasobserved to generate the second and third harmonics of Nd:YAG laser inthe same direction. An Nd_(0.12)Gd_(0.16)Y_(0.72)COB crystal also, isone of such crystals.

As described in detail heretofore, the present invention providesGd_(x)Y_(1−x)Ca₄O(BO₃)₃ constituting a solid solution having ahomogeneous composition, and an oxyborate crystal of plural types ofrare earth elements as a wavelength conversion nonlinear optical crystalhaving superior optical properties, whose double refraction index can becontrolled.

Moreover, the present invention also provides a novel nonlinear opticalcrystal for second harmonics generation, and a method and apparatus forgenerating second harmonics using this nonlinear optical crystal.

What is claimed is:
 1. A wavelength conversion crystal represented by aformula (I), M¹ _(x)M² _(1−x)Ca₄O(BO₃)₃, wherein each of M¹ and M²represents one or more types of different rare earth elements selectedfrom the group consisting of Gd, Y, La and Lu, and 0<x<1.
 2. A methodfor generating a laser beam comprising performing wavelength conversionof light using the wavelength conversion crystal of claim
 1. 3. Anapparatus for generating a laser beam comprising the wavelengthconversion crystal of claim
 1. 4. A nonlinear optical crystal for secondharmonics generation represented by a formula (II),Gd_(x)Y_(1−x)Ca₄O(BO₃)₃, wherein 0.01≦x≦0.35.
 5. A method for generatinga laser beam comprising performing wavelength conversion of light usingthe wavelength conversion crystal of claim
 4. 6. An apparatus forgenerating a laser beam comprising the wavelength conversion crystal ofclaim
 4. 7. The apparatus for generating a laser beam of claim 6,wherein a nonlinear optical crystal is used as means for generatingthird harmonics.
 8. An apparatus for generating a laser beam comprisinga nonlinear optical crystal element, which is a calcium oxyborate-typecrystal comprising Gd and Y, to which Yd or Nd is doped, said nonlinearoptical crystal element performing both the laser oscillation of afundamental wave and generation of laser beams of second and thirdharmonics.
 9. The apparatus for generating a laser beam of claim 8,wherein said calcium oxyborate-type crystal is represented by a formula(III), Gd_(x)Y_(1−x)Ca₄O(BO₃)₃, wherein 0<x<1, and Yb or Nd is dopedthereto.
 10. The apparatus for generating a laser beam of claim 8,wherein said calcium oxyborate-type crystal comprises rare earthelements other than Gd and Y to substitute Gd or Y.
 11. An apparatus forgenerating an ultraviolet laser beam comprising the apparatus of claim8, wherein said apparatus of claim 8 comprises a wave plate and areflective member for semiconductor laser excitation.
 12. A method forgenerating a laser beam comprising performing wavelength conversion oflight using the wavelength conversion crystal of claim
 1. 13. Anapparatus for generating a laser beam comprising the wavelengthconversion crystal of claim
 1. 14. The apparatus for generating a laserbeam of claim 9, wherein said calcium oxyborate-type crystal comprisesrare earth elements other than Gd and Y to substitute Gd or Y.
 15. Anapparatus for generating an ultraviolet laser beam comprising theapparatus of claim 10, wherein said apparatus of claim 10 comprises awave plate and a reflective member for semiconductor laser excitation.